The thermal evaporation and condensation of solid materials such as metals, ceramics and inorganic compounds to form layers is a relatively developed art. There are many sophisticated prior an techniques and apparatuses which permit such materials to be evaporated from a source and condensed to form a layer on a substrate displaced a distance from the source. The processes all involve heating a material to be evaporated to a temperature at which it has a significant vapor pressure, thus creating a vapor stream. Heating techniques include direct methods, such as heating the material to be deposited directly using resistance, induction, electron beam or laser heating means to melt all or some portion of the material to be evaporated, or indirectly, such as by heating the surface of a higher melting material and flashing the material to be evaporated off the hot surface. However, for many materials, such as multi-constituent metal alloys, these methods do not work well because they do not produce a condensate having a uniform composition through its thickness which closely resembles the starting material. This is because the rates of evaporation of materials comprising a plurality of elements are related to their vapor pressures at the temperature of the evaporation source. In the case of alloys, particularly multi-constituent alloys, one or two elements typically have significantly higher vapor pressures than the others, such that the condensate is richer than the starting material in these elements. If the material being evaporated has a fixed volume and is entirely evaporated, the condensate will have a non-uniform composition throughout its thickness, but will reflect, in a macroscopic sense, the starting composition of the material. If the starting material is continually replenished, such as by maintaining a constant pool volume, the composition of the condensate will be higher throughout its thickness in the elements which have higher vapor pressures.
Evaporation has been used extensively for some applications, including the manufacture of thin solid films for use as sensors, circuit metallizations, dielectrics and other components utilized in solid state electronic devices, and for many other purposes. The thicknesses of these thin films are usually measured in microns (e.g. about 10.sup.-1 -10.sup.2 microns for many electronic applications). However, evaporation has not been widely used to produce structural coatings or structural members that have greater thicknesses, thicknesses that would typically be measured in millimeters or centimeters, for several reasons. One reason is that the high rates of deposition required to make evaporation economically practical are difficult to obtain with conventional evaporation methods ,and apparatuses. Electron beam evaporation can be used to produce high rates of evaporation for many pure metals and alloys, but the application of high beam currents to the materials to be evaporated is known to cause significant problems such as splattering. Splattering is thought to be due to the creation of metal vapors under the surface of the molten metal by application of the electron beam. Splattering occurs when such vapors expand rapidly and are ejected violently from the surface of the molten metal along with small molten droplets. These droplets often are deposited along with the condensate, creating defects in the condensate. A second reason that evaporation is not widely utilized for structural materials is related to compositional control. As noted, in materials comprising a plurality of elements, the various elements are usually evaporated at different rates related to their vapor pressures at the temperature of the melt from which they are evaporated and the composition of the melt. Depending on the elements, these vapor pressures can differ significantly. Thus, the development of a uniform condensate composition requires extensive characterization of the evaporative characteristics of the material. A third reason, also related to compositional control, is that during evaporation the melt is being preferentially depleted of certain of the elements due to their differing evaporative characteristics. Thus, it is usually necessary to replenish the material being evaporated to prevent the composition of the condensate from changing during the course of a deposition. Deposition of a material with a uniform composition requires the control of many variables. Small changes in the electron beam pattern employed to melt the alloy and maintain the molten pool will often dramatically alter the composition of the vapor and, therefore, of the condensate formed from the vapor. In addition, small changes in the power supplied and ingot feed rate also result in changes in the vapor composition. Further, growth of condensate on the edge of the pool often has a strong effect on the vapor composition. Generally speaking, current evaporation processes are not robust insofar as the evaporation of multi-constituent materials from a single source are concerned, in that they are not easily adapted to compensate for or overcome the deleterious effects of changes in various process variables.
Up to this time, the major advances in evaporation have been in making the adjustable parameters more and more controllable. For example, a laser beam with a feed-back loop can be used to automatically control the feed rate of an ingot into the evaporative pool thus assisting in maintenance of a constant liquid volume in the pool. This maintenance of constant pool volume tends to stabilize the composition of the vapor stream and of the condensate formed from the vapor stream. However, even with advances in certain of the control mechanisms, it has been found that in general the evaporation rates must be kept low in order to obtain adequate compositional control.